1. Field of the Invention
The present invention relates to an optical deflector that deflects radiation beams.
2. Discussion of Related Art
An optical deflector is used to improve data access time in CD players or magneto-optical disk drivers and also utilized in laser printers, scanners, or display devices. A prism is a typical example of the optical deflector that deflects beams from its original direction. This prism is mostly used with respect to collimating beams in a free space.
A prism-type optical deflector may also have a waveguide shape and be formed of silica, GaAs, InP, LiTaO3, or polymer, etc. In this case, an optical deflector having the waveguide shape refers to a structure that guides beams in a vertical direction and deflects the beams in a lateral direction.
The principle on which the beams are deflected is as follows. When the beams propagate in a certain pattern, a refractive index of a medium in the pattern is changed by an external electric signal. Thus, the beams are deflected according to a refraction characteristic relative to an angle on which the beams are incident at an interface between a region where the refractive index is changed and a region where the refractive index remains unchanged.
Meanwhile, the deflection of beams radiating in a lateral direction through a waveguide can be employed in various optical devices, such as optical switches, arrayed waveguide gratings, and concave gratings. Also, since the waveguide can be monolithically integrated with a light source, it is possible to manufacture very small optical devices. Conventional waveguide-type optical deflectors are discussed in detail in Reference document 1 [IEEE Journal of Lightwave Technology, vol. 13, no. 15, October 1995] and Reference document 2 [IEEE Journal of Lightwave Technology, vol. 12, no. 8, August 1994]. Reference document 1 makes an analysis of beam propagation in terms of wave optics to show that beams are deflected by a waveguide type optical deflector, and Reference document 2 teaches the optical deflection characteristics of an optical deflector by the help of experiments using gas laser (He—Ne, 635 nm).
However, the conventional optical deflectors are applied to incident beams that are not radiation beams but collimating beams. Hereinafter, the problem of the conventional optical deflectors will be described in detail with reference to FIGS. 1A and 1B. FIGS. 1A and 1B are conceptual diagrams for explaining the optical deflection characteristics of a conventional optical deflector.
FIG. 1A illustrates the optical deflection characteristics of a triangular-type optical deflector 20 when beams that are transmitted through an optical waveguide (not shown) and incident on the triangular-type optical deflector 20 are collimating beams. Referring to FIG. 1A, since all beams have the same incident angle as an interface, they are refracted and deflected at the same angle. As shown in FIG. 1A, the beams, which propagate in parallel in an x direction in a region 5 having an effective refractive index of n1, are deflected by the triangular-type optical deflector 20 having a predetermined pattern and an effective refractive index of n2. Here, the effective refractive index refers to an effective refractive index of the optical waveguide and varies with a change in the refractive index of a medium in a core layer of the optical waveguide.
FIG. 1B illustrates the optical deflection characteristics of the triangular-type optical deflector 20 when beams that are transmitted through the optical waveguide 10 and incident on the triangular-type optical deflector 20 are beams radiating from the point O. In FIG. 1B, the beams that radiate from the point O are denoted as A, the beams in a pattern of an optical deflector are denoted as B, and the beams that have passed through the pattern of the optical deflector are denoted as C. The radiation beams A pass through an interface between regions having refractive indexes of n1 and n2 and are refracted at an angle of θ2 according to Snell's law. The beams B pass again through the interface between the regions having refractive indexes of n2 and n1 and are refracted at an angle of θ3 to θ4. As a result, the beams A are refracted twice and become the beams C. Accordingly, as can be seen from FIG. 1B, because the radiation beams A are incident on the interface at respectively different incident angles, the radiation beams A are refracted in respectively different directions.
In conclusion, it is difficult to apply the above-described conventional optical deflectors to radiation beams.